Many mammalian sera possess nonspecific, heat-stable, cationic bactericides active in vitro mainly against gram-positive bacteria. We have recently isolated the primary bactericidal agent from rabbit serum (PC-III) and have purified it to homogeneity. The role(s) of this antibacterial peptide in the process of infection will be examined in vivo. A second aspect of the proposal is concerned with a detailed investigation of the mechanism of killing of test bacteria in vitro, and another examines the cellular source of the bactericidal peptide. The possible efficacy of this unique antimicrobial as a chemotherapeutic agent in experimental infections will also be scrutinized.